Apparatus for the control of charge on a moving web

ABSTRACT

A method for controlling electrostatic charges on a web, wherein the web is pulled over a metal roller which is connected to a D.C. voltage supply, the electrostatic field of the web is measured, and the voltage supply is controlled as a function of the measured field to produce a voltage of a polarity whereby the electrostatic field produced by the web is greatly neutralized.

United States Patent [1 1 De Geest Jan. 22, 1974 [54] APPARATUS FOR THECONTROL OF 3,521,126 7/1970 Granzow et a1 317/262 A X CHARGE ON A MOVINGWEB 2,980,834 4/1961 Tregay et al 317/262 A 2,826,166 3/1958 Davis 317/2[75] Inventor: Wilfried Florent De Geest, Berchem,

Belgium Prima Examiner--J. D. Miller A i tN.V., M t l, [73] sslgnee gg gfg or Se Assistant Examiner1-1arry E. Moose, Jr.

Attorney, Agent, or Firm-William J. Daniel [22] Filed: May 3, 1971 [21]Appl. No.: 139,745

7 [57] ABSTRACT [30] Foreign Application Priority Data May 4, 1970 GreatBritain 21,365/70 A method for controlling electrostatic charges on aweb, wherein the web is pulled over a metal roller {52] US. Cl. 317/2 R,317/262 A which is connected to a DC. voltage supply, the elec- [51 Int.Cl. 1105f 3/00 trostatic field of the web is measured, and the voltage[58] Field of Search 317/2, 262 A supply is controlled as a function ofthe measured field to produce a voltage of a polarity whereby theelectro- [56] References Cited static field produced by the web isgreatly neutralized.

UNITED STATES PATENTS 3,671,806 6/1972 Whitmore 317/2 R 2 Claims, 2Drawing Figures D.C. SOURCE it A ,4

CONTROLLER 72 ELECTROMETER PAIENIEDJAN 2'? I914 3. 787, 706

. D.C. SOURCE 12 I6 17 1a ELECTROMETER CONTROLLER Fig. I

20 -o.c. SOURCE 1 ELECTROMETER 23 V 2 Fig. 2

APPARATUS FOR THE CONTROL or CHARGE N A MOVING WEB The present inventionrelates to an apparatus for the control of charge on a moving web, inparticular on a moving web which is being wound or unwound.

It is known that insulators, and conductors when not electricallyconnected to the ground, becomes easily charged. This comes about as aconsequence of rubbing against other surfaces. Friction may be involved,or sometimes only contact followed by separation. In the treatment of aweblike material, the electrostatic charging thereof frequently forms aserious problem because, since a web is generally transported over aplurality of rollers, its electrostatic charge may become important.

A common method for discharging or neutralizing a web comprises movingthe web adjacent a plurality of grounded points. When the field producedby the charged web near the tips of the points attains a critical value,the air in the small region near the tip is ionized so that ions areprojected onto the web in such a way as to neutralize the excess fieldatevery instant. Said method shows the disadvantage that for wide webs agreat number of pointsis required. The points are liable to deformationwhen a discharge unit is manipulated and they may damage a swinging web.In addition, the critical value of afield which is capable of producingionization is about 500. V/cm so that a complete discharge of a web bymeans of said method is not possible.

Another method for rendering the surface of a moving. web electricallyneutral comprises removing charges from the web by intimately contactinga grounded conductor to the opposed surfaces of the web. Further, thismethod is not capable of completely discharging a web because, mainly asa consequence of contact potential effects, it is not possible to reducethe charge on a web below 200 V/cm.

In the treatment of webs, and in particular in the winding of webs, ithas been found that the electrostatic field strength ofa roll of webwhich is being wound may easily amount to kV/cm when the web is notsufficiently discharged, that is below 200 V/cm, prior to its winding.The same effect may occur with a roll of web which is being unwound,with this particularity that the roll field may be zero or practicallyso when the unwinding is started and that it may gradually increase asthe web is unwound. As a consequence of all this uncontrolled dischargesmay occur as a roll is approached by an operator or a grounded object,and said discharge may have deleterious effects, particularly in thecase of light-sensitive photographic material.

It is an object of the present invention to provide an arrangement forthe control of charge on a moving web, whereby the mentioned adverseeffects may be avoided.

According to the present invention, the charge on a moving web iscontrolled by:

a. moving the web with at least one side in intimate contact over acontrol electrode which is electrically insulated from the ground andwhich is connected to a source capable of producing a variable D.C.potential,

b. measuring the electrostatic field produced by the web at a locationclose to the said electrode, and

c. controlling the said source as a function of the measured field so asto produce a D.C. potential of i a magnitude and a polarity which iscapable of substantially reducing the field produced by the web.

According to a preferred embodiment of the invention, the measurement ofthe electrostatic field is done at a location which is between the saidcontrol electrode and the winding station, or between the said electrodeand the unwinding station, if the charge on the web is to be controlledin connection with a winding, or an unwinding, respectively.

If the measurement of the field is done at a location which isdownstream of the control electrode according to the web advance, thesource of D.C. potential may be controlled in such a way as to obtain azero field measurement.

The invention will be described hereinafter with ref erence to thedrawings, wherein:

FIG. 1 is a diagrammatic view which illustrates the apparatus accordingto the invention for the charge control of a web which is being woundup, and

FIG. 2 is a diagrammatic view which illustrates the apparatus accordingto the invention for the charge control of a web which is being unwound.

In FIG. 1 a web 10 is passed over a grounded metal roller 11 before itis wound onto a roll 12. The web 10 may be a photographic support suchas a triacetate or polyethylene terephthalate film which has been passedover a plurality of rollers during its coating and drying and which hasbecome charged as a consequence of its passage on said rollers. Theroller 11 may be the last of said plurality of rollers.

- A common value of the field strength which may be measured at thelocus indicated by the arrow 13 amounts to 200 V/cm. The field strengthmeasured at the roll 12, on the contrary, may be comprised between 1 and5 kV/cm. This latter field strength may also be measured at a locuscomprised between the roll 12 and the grounded roller 11, for instanceat the locus indicated by the arrow 14. It should be understood that thecharge on the wound web gradually increases and that a field strength ofl to 5 kV/cm is only attained near the termination of the winding. It isprecisely at this moment that the roll of wound web must be manipulatedin order to cut the trailing end of the web and to remove the roll fromthe winder.

The charge on the wound web is considerably reduced if the charge iscontrolled by an arrangement which comprises the measuring electrode 15,the electrometer 16, the controller 17, the course 18 which is capableof producing a D.C. output voltage which is a function of the outputsignal of the controller 17, and a control electrode 1.9 in the form ofan electrically conductive roller which is electrically insulated fromthe ground. In the drawing, the roller 19 has been drawn in tangentialcontact with the web 10 because hereinbefore the linear web path hasbeen considered between the roller 11 and the roll 12, but it should beunderstood that the roller 19 will be placed so that the web contactsthe roller 19 over an angular part of its periphery, so that an intimatecontact occurs between the web and the said roller. In addition, meansshould be provided for controlling the web path from the roller 19 tothe electrode 15, so that the distance between the web and the electrode15 remains constant as the diameter of the roll 12 increases duringwinding.

The measuring electrode 15 is an elongated conductor which extendstransversally of the webpath and is separated therefrom, and which isconnected to an electrometer 16. Actually, the electrode 15 forms withthe electrometer. 16 a field strength meter.

The electrometer 16 produces a D.C. output signal which corresponds inmagnitude and polarity with the measured charge on the web. A controller17 amplifies the output signal of the electrometer l6 and controls thesource 18 in such a way that a D.C. voltage is applied at the controlelectrode 19 which is capable of reducing the field produced by the web;

it should be understood that the charge on the web which determines thefield at the electrode 15, is determined by the charge which is presenton the web after its passing over the roller 11, and also by the chargewhich is being accumulated or produced on the roll 12.

The control of the charge onthe web occurs by the injection of electronson the lower web surface by the roller 19 if the field measured at theelectrode 15 is positive, or by the corresponding withdrawal ofelectrons if the measured field is negative. Said field measured at theelectrode 15 may be caused by a determined surplus or lack of chargecarriers on only one surface of the web, but said field may also resultfrom different charge conditions on the different surfaces of the web.The measuring electrode actually measures the resulting field at thelocus of said electrode, and it controls the control electrode 19 inorder to modify the charge on the web in such a way that at the locus ofthe measuring electrode the field is considerably reduced, or evenbecomes zero. Thus, it will be understood that during the winding of theweb the charge on each side of the web is not necessarily removed orneutralized, but that the overall charge is controlled in such a waythat an uncontrolled charge on the web which may cause excessive fieldstrengths at the winding station, is

. avoided.

Whereas formerly field strengths of kV/cm and even higher could bemeasured on the roll 12 of the wound web, it has been shown that thedescribed apparatus permits to control the charge on the web in such away that the field at the roll 12 remains within acceptable limits, thatis lower than 500 V/cm.

ln the arrangement according to FIG. 2, a web 25 is unwound from a roll20 and is passed over two electrically conductive rollers 21 and 22,which are insulated from the ground.

The electric insulation of the roller 21 in respect of the groundamounts to ohms and the roller is connected to the input of a vacuumtube electrometer 23. The output current of the electrometer controls asource 24 which is capable of producing a D.C. output varying between 0and 1,000 volts. In the present case, the polarity of the D.C. output isfixed because it is admitted that the arrangement is used for thecontrol of charge on webs of the same type, and in such case themagnitude of the field increases as the web moves, but the polaritythereof does not vary. The described arrangement permits to controleffectively the charge on the web, and it has been found that at theunwinding of the web any difficulty with respect to excess fieldstrengths is avoided, and this in particular for webs which have beenpreviously wound without any control of the charge thereon.

The present invention is not limited to the described embodiments.

The apparatus according to the invention may also be used for thecontrol of charge on moving webs at other locations than near a windingor unwinding station. Thus, the web charge may be controlled nearcoating stations, laminating stations, drying stations, etc.

The charge on a web may be controlled by measuring the electrostaticfield at both the web sides and by controlling independent controlelectrodes at either side of the web. Both said measurements may occurat locations which are close to each other, but it is also possible tomeasure the field on one side of the web, for instance at an unwindingstation for the web, and to measure the field on the other web side atthe winding station.

We claim:

1. Apparatus for reducing the electrostatic charge on a web moving alonga path which comprises:

a. a conductive electrode roller having a smooth rigid surface anddisposed along said web path with the surface thereof in direct contactwith one surface of said web, said electrode roller being electricallyinsulated from ground,

b. a source of variable D.C. potential connected to said electroderoller,

c. means for measuring the electrostatic field produced by the web at alocus adjacent said electrode roller and producing a signalcorresponding to the magnitude of an electrostatic charge on said web,and

d. a controller for said D.C. source, said controller receiving saidsignal from said field measuring means and controlling said source toapply to said electrode roller a D.C. potential of a polarity oppositeto the measured field and a magnitude sufficient to substantially reducethe charge on the web,

said web having the opposite surface thereof free of contact with anelectrode roller in the region of said conductive roller.

2. Apparatus according to claim 6 wherein said field measuring means isdisposed downstream of said electrode roller.

1. Apparatus for reducing the electrostatic charge on a web moving alonga path which comprises: a. a conductive electrode roller having a smoothrigid surface and disposed along said web path with the surface theReofin direct contact with one surface of said web, said electrode rollerbeing electrically insulated from ground, b. a source of variable D.C.potential connected to said electrode roller, c. means for measuring theelectrostatic field produced by the web at a locus adjacent saidelectrode roller and producing a signal corresponding to the magnitudeof an electrostatic charge on said web, and d. a controller for saidD.C. source, said controller receiving said signal from said fieldmeasuring means and controlling said source to apply to said electroderoller a D.C. potential of a polarity opposite to the measured field anda magnitude sufficient to substantially reduce the charge on the web,said web having the opposite surface thereof free of contact with anelectrode roller in the region of said conductive roller.
 2. Apparatusaccording to claim 6 wherein said field measuring means is disposeddownstream of said electrode roller.